Should I study Combinatorics and Probability more?

If you're shooting for a top score in quant, you shouldn't leave any major holes in your preparation. Combinatorics and probability can certainly be difficult. We've even made them optional in our course, but then, most of our students aren't targeting 49-50 in Quant. Even at that level, you shouldn't expect to see too many such problems--I think I saw one combinatorics problem on my last official test (Q50)--but it's worth putting some time in (without focusing obsessively on this one area). It's impossible to say how skipping these questions will affect your score, as it depends so much on what happens before/after. Even at Q50, you're still going to be missing questions. If you're very strong elsewhere, it may not be so bad to miss a combinatorics/probability question, but you should get to the point where you at least have a shot at the answer to most of the questions you see.

It's also worth noting that probability problems can often be narrowed down through estimation. Is the even in question likely or unlikely? How much so? If there are several events, is the combined probability higher than any individual probability (OR = add) or lower (AND = multiply)? I've even seen official questions in which one or more of the answer choices presented probabilities greater than one. You can cut those immediately! So if you find it too hard to become a probability master, at least you can, er, increase your probability of guessing the right answer!